1. Field of the Invention
The present invention relates to an LED driving circuit.
2. Description of the Related Art
A clock having a radio reception function and digitally displaying time by LEDs is in wide use (see, e.g., Japanese Patent Application Laid-Open Publication No. 1994-21839). FIG. 5 depicts an example of a time display unit of such a clock. The time display unit 100 has a plurality of segments 110 to 116, etc., for digitally displaying a time, each of which is turned on by one LED. For example, out of seven segments 110 to 116 for displaying a ten-digit number in minute, four segments 110 to 113 are turned on to put “4” in display, or, two segments 112 and 113 are turned on to put “1” in display.
In this manner, an LED driving circuit is used as a circuit that controls driving of a plurality of LEDs. FIG. 6 depicts a general configuration example of an LED driving circuit. The LED driving circuit 120 is an integrated circuit that includes a plurality of drive control circuits 121, 122, etc., and a plurality of connection terminals T11, T12, etc. In the LED driving circuit 120, to reduce the number of terminals, one connection terminal is provided for every two LEDs. For example, the connection terminal T11 is provided for LEDs 130 and 131, and the connection terminal T12 is provided for LEDs 132 and 133. A drive voltage COM1 is applied to the anodes of the LEDs 130 and 132 via a resistance R11, and a drive voltage COM2 is applied to the anodes of the LEDs 131 and 133 via a resistance R12.
FIG. 7 depicts an example of the drive voltages COM1 and COM2. The drive voltages COM1 and COM2 are, for example, obtained by rectifying an alternating voltage AC having a frequency of 50 Hz through half-wave rectification, and have phases different from each other by 180 degrees. When LEDs driven by the drive voltage COM1 are LEDs belonging to an A group and LEDs driven by the drive voltage COM2 are LEDs belonging to a B group, the LEDs in the A group and those in the B group are driven alternately. For example, the LEDs 130 to 133 correspond in increasing order to the segments 110 to 113 of the time display unit 100, respectively. For example, the LEDs 130 and 132 in the A group and the LEDs 131 and 133 in the B group are driven alternately at a frequency of, for example, 50 Hz to make a visual display of “4”.
In this manner, according to the LED driving circuit 120, the LEDs are divided into two groups, and are driven by time-division driving. In each group, an LED corresponding to a time to display out of a plurality of LEDs is turned on, so that the number of LEDs to be turned on varies depending on the time to display. As the number of LEDs being on increases in the group, therefore, current flowing through each LED in the group decreases to reduce luminance. Thus, if the number of LEDs to be turned on is different between both groups depending on a time to display, luminance given by LEDs in the A group and that given by LEDs in the B group becomes different from each other, which results in luminance irregularity in time display.